1. Field of the Invention
The present invention relates to a transfer apparatus which includes a transfer material carrying member composed of a carrying member frame, a semiconductor layer stacked on a surface of the carrying member frame, and a conductive (dielectric) layer covering the semiconductor layer, and transfers a toner image formed on an image carrier to a transfer material carried on the transfer material carrying member by electrostatic attraction. Specifically, the invention relates to a transfer apparatus for use in a color image forming apparatus such as laser printers, copying machines, and laser faxes.
2. Description of the Related Art
With conventional color image forming apparatuses, a plurality of toner images of respective colors are formed on an image carrier composed of a photoreceptor, and the toner images of respective colors are transferred in a superposed relation onto a single transfer material, whereby a full-color image is formed.
A transfer apparatus for use in the color image forming apparatuses as described above is disclosed in Japanese Unexamined Patent Publication JP-A 5-173435. The transfer apparatus includes a transfer material carrying member composed of a carrying member frame, an elastic layer such as urethane foam covering the carrying member frame, and a conductive layer such as PVDF (polyvinylidene fluoride) covering the elastic layer. This results in the formation of a gap of 10 .mu.m or more between the surface of the elastic layer and the underside of the conductive layer. With the transfer apparatus, the transfer material carrying member is caused to carry a transfer material by electrostatic attraction, and the transfer material carrying member is applied with a voltage of an opposite polarity to that of a toner. Thus, toner images of respective colors are successively transferred in a superposed relation to the carried transfer material.
Also in Japanese Unexamined Patent Publications JP-A 4-278978 and JP-A 4-335683, there are disclosed image forming apparatuses characterized by their respective transfer apparatuses. Each of the transfer rollers which the respective transfer apparatuses in the image forming apparatuses in both the publications have is so constructed that a foamed material with a predetermined thickness which has a conductivity is disposed around the transfer roller shaft. In the direction of the thickness of the foamed material of the image forming apparatus in JP-A 4-278978, at the closer position the cells are present to the contact surface between the foamed material and the transfer roller shaft, and the outer surface of the foamed material, the more they increase in size or number. Also, in the direction of the thickness of the foamed material of the image forming apparatus in JP-A 4-335683, at the closer position cells are present to the outer surface of the foamed material, the more they increase in size.
With the transfer apparatuses in the three publications, in the case where carrying of a transfer material onto the conductive layer by electrostatic attraction is given priority, it is required to increase the gap in size. When the gap is increased in size, there arises a problem as follows: that is, in a transfer area for transferring toner to the transfer material, within the outer surface of the conductive layer and the outer surface of the foamed material, the amount of toner transferred by the gap portion is different from the amount of toner transferred by the gap-free portion, and hence the formed image results in the image having mottles, i.e., transfer nonuniformity.
Further, resin sheets such as OHP sheets used as transfer material does not tear easily as compared with paper. Accordingly, it is required to increase the gap in size to such a size as to entail a problem in image quality for carrying such a resin sheet by electrostatic attraction with reliability.
Also, in the case where the gap is decreased in size for giving priority to image quality, the performances of carrying the transfer material by electrostatic attraction becomes insufficient, especially, the attraction force at high temperatures and high humidities is reduced. For example, there occurs a problem that when a plurality of toners of respective colors are transferred at the time of forming a color image, the transfer material is peeled off during transfer.